I'm in the market for an SBR. Make and manufacturer are undetermined at this point. I have some ideas, but they are relatively uninformed ideas, so I'm hoping this thread can help lead me the right direction without devolving into a fan-boy slug fest.

Basic Want List: Not too extensive at the moment.

AR Based: My thought is it is about the most common platform on the planet, along with the AK, but in this country it's probably top dog. Accessories/parts are around every corner.

Compact: I am perfectly happy with a 7"-10" barrel unless there is some overriding reason not to. I am not interested in a 500mtr tac driver. As long as I could get the job done within, say 200 mtrs max I'd be happy. I intend to add a can to keep the sound level somewhere just short of lethal. The barrel needs to be on the short side to keep the overall package compact.

Caliber: My first thought is to go with the 5.56 since it is relatively effective and it is relatively doable to carry it in bulk. I had thought of something in 9mm given that I'm not looking for extreme range, but I am concerned about parts compatibility across the platform. I felt it best to stick with the standard.

The rest is stuff I'm not sure about.

Gas piston or DI? I'd read that the piston system was cleaner, cooler, and more reliable. The last bit especially in a short barrel system. OTOH, builders like Noveske make some pretty short setups using the DI system. What's the for Dummies explanation of the differences in the two, especially as it relates to an SBR that may or may not have a can on it at any given time.

AR, M4....What gives? What exactly are the differences here and does it even matter given what I'm looking for?

I've also noticed platforms advertised as having a full auto bolt/carrier what not and wondered what difference this made since new manufacture F/As are not allowed for the general public. Is it just a durability issue, and if so what other "full auto" pieces/parts should I be looking for in a build?

This kind of leads me to an unrelated question that is strictly a curiousity thing. What exactly is the difference between a semi auto AR and a full auto version? I've never looked it up specifically but bits and pieces I've read here and there made it seem the difference were slightly more than minor which confused me. Explanation below.

When I was in the Marine Corp we had the old style M16. I remember specifically that they shot semi-auto only, even though marked as full auto capable. When they wanted us to be able to shoot full auto the only thing they gave us was a different selector switch. We popped out the old selector, popped in the new one, and it was ready to rock and roll. If I remember correctly the full auto selector had a much simpler shape than the semi only selector. IE, the full auto one was much simpler to machine than the other one. Anyway, recent reading made it seem more complicated than that, hence my curiousity.

More than happy to answer any pertinent questions related to the platform I'm looking for, as well as entertain any competing suggestions. Thanks a bunch.

The full auto BCG in most premium ARs is in fact for durability and reliability.

There is a big difference between the AR15 and M4/M16 receivers. Semi-auto receivers are designed not to take the M4/M16 auto sear without significant machining. The most immediate difference between the the two receivers from the outside is that an M4/M16 has a third pin for the auto sear. Internally, you will notice that the rear of an AR15 receiver is solid. There is a big cut in the rear of an M4/M16 receiver for the auto sear to function. The other difference you will notice is that the full auto hammer has a hook to catch the auto sear. The semi-auto hammer has no hook. Part of the auto sear's job in closed bolt select fire weapons like the AK and M4 to prevent an out of battery firing that could blow up the receiver. There is also a high shelf in many ARs that prevents the use of an auto sear and drop in conversion devices.

Gas piston or DI? I'd read that the piston system was cleaner, cooler, and more reliable. The last bit especially in a short barrel system. OTOH, builders like Noveske make some pretty short setups using the DI system. What's the for Dummies explanation of the differences in the two, especially as it relates to an SBR that may or may not have a can on it at any given time.

The M16 was originally designed with the idea you would have a 20" rifle with a gas port at about 14" or so down the barrel. This means that the port pressure would be approximately 12k-15k PSI and that the dwell time would be whatever time it took for the bullet to travel the remaining 6" or so and exit the barrel.

When the barrel is shortened in a direct impingement system, you have a couple of different issues: 1) You still want dwell time to be close to the original; because you are using the same M16 parts in the receiver. 2) You are tapping pressure further back in the barrel, meaning the port pressure is higher.

Normally, what you would do is enlarge the gas port (or reduce it) so that you tap off enough gas to cycle the action with whatever dwell time you have. However, when you add a suppressor to the mix, you are adding a device that adds an extra 6" or so of dwell time and is purposely designed to slow the exit of gas from the rifle - which means that the same gas port that works with a 10.5" unsuppressed barrel is now overgassed when the suppressor is attached. Likewise, if you design it for use with the suppressor, it will be undergassed when the suppressor is not in use.

A gas piston is a little less susceptible to these issues, although it will still be affected by them. It won't necessarily be more reliable though and since a suppressor will blow a lot of gunk back into the action regardless, it isn't going to be dramatically cleaner than a suppressed DI.

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AR, M4....What gives? What exactly are the differences here and does it even matter given what I'm looking for?

The differences aren't really relevant for your purposes. AR or Ar15 can refer to any of the many dozens of variants of the M16 family of weapons. An M4 is a specific variant with specific features - it is what is currently the general issue rifle for the Army, although the term will also be used to describe M4 look-alikes that are semi-auto (also called M-4geries).

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've also noticed platforms advertised as having a full auto bolt/carrier what not and wondered what difference this made since new manufacture F/As are not allowed for the general public. Is it just a durability issue, and if so what other "full auto" pieces/parts should I be looking for in a build?

The differences between a full-auto bolt carrier and a shrouded semi-auto bolt carrier are mainly in how long it holds the hammer down as the bolt carrier cycles. As long as the firing pin on the bolt carrier is shrouded, both should be equally reliable in a semi-auto build, though the full-auto will be a touch heavier which may delay unlocking a few microseconds more.

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This kind of leads me to an unrelated question that is strictly a curiousity thing. What exactly is the difference between a semi auto AR and a full auto version? I've never looked it up specifically but bits and pieces I've read here and there made it seem the difference were slightly more than minor which confused me. Explanation below.

Given your requirements, you might take a look at a 9" AR upper in .300 Blackout (google "AAC Honey Badger"). I haven't ever played with one and marketing hype is just that; but according to the marketing hype, the .300 Blackout was designed for the exact role you are looking at. Only issue is that cost of ammo would be significantly higher than 5.56mm.

Assuming the marketing hype is true (big assumption), I would recommend it over 5.56mm for the following reasons:

1. A 7-10" barrel on a 5.56mm AR is difficult to get running well for all but the most experienced makers. The same length barrel AND occasional suppressor usage is going to be a challenge.

2. The 5.56mm relies on velocity to be effective. 10" and less barrel means you are sacrificing most of what makes 5.56 so useful. You lose velocity but you keep the same small projectile.

By contrast, the .300 Blackout was designed from the get-go to be fired from short (12" and less) barrels in direct impingement ARs with both suppressed and unsuppressed use. In addition, under 200m, it is going to offer more energy because it will have a bigger bullet and it is designed to make the most out of shorter barrels. You'll also have the option of using 220gr subsonic rounds that will make the rifle as quiet as an MP5SD (at the expense of terminal performance and a rainbow like trajectory even at 200yds).

Again, I haven't actually verified any of those claims or even taken time to consider whether they could possibly be true. I'm relying totally on marketing hype unfiltered by any skepticism at all.

By contrast, the .300 Blackout was designed from the get-go to be fired from short (12" and less) barrels in direct impingement ARs with both suppressed and unsuppressed use. In addition, under 200m, it is going to offer more energy because it will have a bigger bullet and it is designed to make the most out of shorter barrels. You'll also have the option of using 220gr subsonic rounds that will make the rifle as quiet as an MP5SD (at the expense of terminal performance and a rainbow like trajectory even at 200yds).

I had read a bit about the .300, but had discounted it because it was stepping away from what was "common". One of my goals is to have a platform with relatively common parts and readily available caliber.

From a form factor perspective this is about exactly what I'm looking for...

I have no idea if this company is reputable or not, nor the quality of their gear. Up to now I've been looking at Noveske or the new Barrett entry. I know they are considered of good quality, but I wonder how much of the price is based just on the name.

I don't really want to build one, at least not yet. I like the idea of it coming from the factory already "made" and thereby avoiding that issue, and having to engrave my life history on the receiver.

The AR platform is a good starting point for you till you know what you want.
The receiver is the gun, so different configurations and calibers and platforms do not require additional firearm registration.

If you want a factory gun, then you can't form 1 and build yourself, you will have to hook up with a NFA dealer to order you one.

For a newbie I would get a facytory Rock River Arms short rifle.

This will get you started, you can change configuration later. When you order the rifle get a silencer on order for it also, an 11.5" sbr with silencer will be the same size as most folks semi auto carbine.

Cousino Firearms in Toledo is who I reccomend, but you probably have a dealer closer.

As for piston system, they all work fine, except there is not std. like gas gun. Every body and his brother has parts for your gas gun because it is industry std.

Unless Uncle Sugar's army would standardize on the particular piston design then what ever system you would choose would limit your parts availability to that one custom manuf. and parts would only be viable as long as that company chose to make them.

I had read a bit about the .300, but had discounted it because it was stepping away from what was "common". One of my goals is to have a platform with relatively common parts and readily available caliber.

If you are looking for common parts, you probably do not want a piston gun since most of them have proprietary parts and every piston system is slightly different from the other guy's piston system.

As for a readily available caliber, it will probably be a while until you see .300 Blackout in WalMart. On the other hand, Federal, Remington, Hornady, etc. all sell the caliber online. I think the best price I've seen in a quick look is still $0.08 per round though.

And the only difference parts-wise between a .300 Blackout and 5.56mm is the barrel/barrel extension. It uses the same gas tubes, bolts, magazines, etc.

But I definitely understand the desire to stick with 5.56mm.... I just think you are sacrificing a lot of what makes 5.56mm useful once you get down into barrels under 11.5" though.

If you are looking for common parts, you probably do not want a piston gun since most of them have proprietary parts and every piston system is slightly different from the other guy's piston system.

That changes things a bit. Obviously non-standard parts would be an issue given my priorities. What about these switched gas blocks I've read about? I'm assuming this is going to be proprietary as well. If I recall correctly these are made specifically for running a weapon silenced/unsilenced by controlling the speed of the cycling, correct?

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As for a readily available caliber, it will probably be a while until you see .300 Blackout in WalMart. On the other hand, Federal, Remington, Hornady, etc. all sell the caliber online. I think the best price I've seen in a quick look is still $0.08 per round though.

.08, or .8?

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And the only difference parts-wise between a .300 Blackout and 5.56mm is the barrel/barrel extension. It uses the same gas tubes, bolts, magazines, etc.

So if I had a 5.56 I could just swap out the barrel for a .300 and be good to go? Or to be really simple two separate uppers of each caliber and swap out as desired. That might be an interesting angle.

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But I definitely understand the desire to stick with 5.56mm.... I just think you are sacrificing a lot of what makes 5.56mm useful once you get down into barrels under 11.5" though.

Can you explain this in a bit more detail? I've watched vids (yeah, I know) where the makers are making center mass out to 300 yrds, with barrels of less than 8". For what I want, I'm good with that. I'm not looking for .5 MOA at 300 yds or anything. If I want that I'd buy a regular rifle, and probably hire someone to do the shooting because I doubt I'm capable of that in any case.

To you other guys. I appreciate the feedback and apologize for not replying directly. Quoting is a bit of a chore on this site.

What about these switched gas blocks I've read about? I'm assuming this is going to be proprietary as well. If I recall correctly these are made specifically for running a weapon silenced/unsilenced by controlling the speed of the cycling, correct?

I'm not sure what you are talking about; but I am assuming you are referring to things like the Noveske Switchblock. These basically allow you to reduce the amount of gas that is fed to the gun to account for the longer dwell time and backpressure created by a suppressor. They are neat and you can reduce action noise by turning the gas off entirely if you want.

At the same time though, the problem with adjustable gas regulators is that they always seem to be adjusted wrong and you get stoppages until you figure that out. My personal preference is just to upgrade the buffer a bit in a direct impingement AR and then monitor wear more closerly and/or replace parts more frequently (say every 6,000 rounds instead of every 10,000 rounds).

The adjustable gas blocks for the AR are proprietary parts, though I haven't ever heard of one needing to be replaced due to wear or breakage. And if you were unable to find one, you would still be able to use the normal gas block.

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.08, or .8?

My mistake, $0.85/round.

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So if I had a 5.56 I could just swap out the barrel for a .300 and be good to go? Or to be really simple two separate uppers of each caliber and swap out as desired.

Assuming the marketing hype is true, that is what they are claiming. Though if you are going to run both calibers, you want to be careful about putting the wrong magazine in the wrong upper.

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Can you explain this in a bit more detail? I've watched vids (yeah, I know) where the makers are making center mass out to 300 yrds, with barrels of less than 8". For what I want, I'm good with that.

If you are just looking at accuracy, then 5.56mm is probably the way to go since it will still be flatter shooting than .300 even in a short barrel. What I was talking about is what happens when the bullet hits an animal.

5.56mm relies on velocity to make its relatively small bullet effective when it hits something and as originally conceived, it was designed around a 20" barrel. When you cut that barrel length in half, you get a lot more noise and flash and the velocity drops. Since F=ma squared, the drop in velocity has a bigger effect on the kinetic energy available to do work (like make big cavities in jello). And if you go to a subsonic round to make the rifle really quiet, you have basically crippled 5.56mm.

.300 BO isn't very fast, so it will have a more arcing trajectory that 5.56mm that will make shooting past 200yds more challenging. However, at closer ranges it will have more mass and it was designed around very short barrels to begin with. So it will have more energy to do work. And because it uses .308 bullets, you have a very wide range of bullets to choose from, including things like 220gr subsonic rounds that can be subsonic; but still have a lot of mass and a decent ballistics coefficient.

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